The present invention pertains to amplifiers for extremely high frequency signals. Advanced communication satellites use millimeter wave or extremely high frequency (EHF) bands to obtain large channel bandwidths for high data rate transmission. Typically, travelling wave tube (TWT) amplifiers are used to produce the required output power in the millimeter wave frequency bands, due to output power limitations of solid state amplifiers. However, TWT amplifiers are large, heavy, and not as reliable as solid state amplifiers. They usually have a limited lifetime in space. High DC power consumption is a primary concern with EHF TWT's, which have power added efficiencies on the order of 10 percent. Similarly, diode amplifiers, such as Impatts or Gunn's, have even lower efficiency.
By contrast, solid state amplifiers with power added efficiencies of 30 to 40 percent can be achieved at EHF (Class B mode) using state-of-the-art hetero-junction bipolar transistor (HBT) technology. However, the output power levels of these devices at EHF are in the 20 to 100 milliwatt range, which is far too low for use as a satellite transmitter. The amount of RF output power that can be obtained from a reliable, efficient, and producible active amplifier is limited to approximately 150 MW because of thermal, space, and other constraints associated with the device when used at gigahertz (GHz) frequencies, such as 60 GHz. Thus, large numbers of devices are required to obtain signals of several watts. A commonly used type of microwave power combiner is the corporate combiner, shown conceptually in FIG. 1. In corporate combiners the RF fields follow transmission line paths as they combine (or split) in series in a binary tree. Amplifiers are bonded into each leg of the tree (requiring 128 RF bonds for a 64-way combiner). The corporate combiner may include driver amplifiers 31 on the signal input, and medium power amplifiers 33 at stages along the signal paths. High power amplifiers 35 create the amplified signal, which is then combined in the output tree.
Corporate combiners can be made in microstrip using the Wilkinson combiners circuits or in waveguide using magic tees. Output power degradation resulting from failed elements is characterized by: P.sub.out =P.sub.total * (1-n/N).sup.2 where N is the total number of elements, P.sub.total is the total power of N elements, and n is the number of failed (or noncontributing) elements. This does not correspond to graceful degradation on a one-to-one basis. A 64-way corporate combiner requires 127 waveguide magic tee sections or microstrip Wilkinson combiner sections. This limits the achievable bandwidth. Furthermore, it does not allow for higher combining levels to produce more output power.
In corporate combiners, loss is a function of the number of elements, since the combiner line lengths necessarily increase with the size of the array. The total loss is A*L; where A is the attenuation per unit length and L is the total path line length.
Typical corporate power combiners, such as Lange couplers or N-way Wilkinsen radial combiners, are very inefficient, especially in combining outputs from a large number of amplifiers. The high impedance transmission lines required for matching are unrealizable for large combining ratios. In addition, failure of a single amplifier causes a large degradation in combiner output power.
Another type of microwave power combiner is a spatial combiner, shown conceptually in FIGS. 2 and 3. Spatial combiners combine the fields in space rather than using transmission lines, using well known antenna principals. The fields are launched from a source 39 with a feed horn 41. A first lens 43 creates a planar phasefront, and MMIC power amplifiers 45 increase the signal level. Uniform energy fields converge to a condensed area (feed horn) 47 using a second lens 49 or a wall structure to guide the fields toward convergence.
In spatial combiners the loss is independent of the size of the array or the number of elements which are being combined. This leads to growth potential since more power can be achieved by merely increasing the size of the array. Losses are only associated with mismatch and transmission through a focusing lens or guided wall structure. Graceful degradation associated with noncontributing array elements is assured since the output power is characterized by P.sub.out 32 P.sub.total *(1=n/N). Note that in this expression the exponential factor 2 is missing.
Spatial combiners do not have the problem of a large degradation of output power due to failure of a single amplifier because the signal paths are in parallel. This principle is used widely in antenna designs. For example, random array element failure in a large array produces only a minimum effect on the antenna gain and antenna pattern. Using spatial combiner for power amplification follows well-known antenna theory. Furthermore, combining efficiency is not limited to any particular number of elements as in the case of the corporate combiners since the combining loss is almost completely independent of the number of elements.
FIG. 3 illustrates a feedthrough spatial combiner having a signal source for the entering signal. A first lens 43 creates a planar phase front for amplification by the array of MMIC power amplifiers. The second lens 49 focuses the amplified signal onto the output port 51. However, there remains the problem of dissipating the heat generated by the MMIC power amplifiers 45.
The concept of using spatial combiners for achieving high output power from a large number of amplifiers has not been widely adapted due to three major problems: 1) Heat generated by the amplifiers cannot be readily dissipated, 2) Fabrication and installation is difficult, complex, and costly, and 3) Routing RF signals to the back side of the array (where the combiner horn is located) requires bonding RF interconnections which degrades reliability and performance at EHF. The present invention resolves these deficiencies.